Hydraulic pump

ABSTRACT

Sufficient pressure is produced in a pocket of a hydrostatic bearing when a corresponding cylinder is about to switch from a suction stroke to a discharge stroke in order to prevent solid contact of sliding faces. A plurality of cylinders are arranged on a circle around a center axis of a cylinder block rotatably supported in a housing. Pistons reciprocates in the respective cylinders. A suction port and a discharge port provided to the housing selectively communicate with the respective cylinders in accordance with a rotational position of the cylinder block. A drive shaft inclined relative to the center axis of the cylinder block, a rotation transmitting mechanism for transmitting rotation of the drive shaft to the cylinder block for rotating the cylinder block synchronously with the drive shaft, and a rotating disk rotating together with the drive shaft and cooperatively engaged with the pistons are further provided. A fixed sliding contact face is formed in the housing to have a sliding contact with a rear face of the rotating disk. Pressure pockets are formed on the rear face of the rotating disk in correspondence with positions of the respective pistons. Pressure paths for conducting hydraulic pressure in the respective cylinders to the respective pressure pockets via the pistons are further provided. There is further provided a pressure introducing mechanism for introducing a high pressure fluid to a pocket moving along the fixed sliding contact face along with the rotating disk at a position in which a corresponding cylinder is about to switch from a suction stroke to a discharge stroke.

TECHNICAL FIELD

This invention relates to a hydraulic axial piston pump supporting arotating disk in a housing via hydrostatic bearings against a reactionforce of pistons.

BACKGROUND OF THE INVENTION

In an axial piston pump, a cylinder block has a plurality of pistons,each piston receives a reaction force in accordance with cylinder innerpressure and the reaction force is transferred to a rotating diskrotating along with the cylinder block. A force corresponding to thereaction of the pistons is exerted between a rear face of the rotatingdisk and the housing, and this force brings about large frictional forceon sliding faces of the rotating disk and the housing.

It is known to provide hydrostatic bearings between sliding faces of therotating disk and the housing in order to reduce the frictional force.The hydrostatic bearing is so constructed that a hole penetrating thepiston is connected with a pocket provided at a sliding face of therotating disk so as to conduct hydraulic pressure in the cylinder to thepocket. The hydraulic pressure in the pocket then acts between therotating disk and the housing, to reduce contact pressure of the slidingfaces and to reduce the frictional force therebetween.

The same number of pockets is provided as that of the pistons, and theinner pressure of each cylinder is conducted to a corresponding pocket.A half of one rotation of the cylinder block corresponds to a suctionstroke with which the cylinder inner pressure becomes low and aremaining half of the rotation corresponds to a discharge stroke withwhich the cylinder inner pressure becomes high. The friction of thesliding faces is changed in accordance with the reaction force of thepistons and is large in the discharge stroke and small in the suctionstroke. Therefore, the cylinder inner pressure conducted to thecorresponding pocket via the through hole of the piston has a magnitudedepending on the reaction force of the piston exerted on the rotatingdisk. Accordingly, large pressure is exerted in a region having largepiston reaction force (discharge stroke) and small pressure is exertedin a region having small piston reaction force (suction stroke) tothereby maintain a balance for the hydrostatic bearing.

However, there is a case in which the high pressure is not exertedimmediately to the pocket at a region of switching from the suctionstroke to the discharge stroke. Although when the suction stroke isswitched to discharge stroke in accordance with rotation of the cylinderblock, the cylinder inner pressure is rapidly increased. However, theremay be a small delay in transmitting this pressure change to thecorresponding pocket. The delay depends on a volume of the pocket or thenarrowness of a transmitting path.

In a transient period of time producing such a delay in response, thereis a concern in which sufficient support force by the hydrostaticbearing is not produced, solid contact (metal contact) is brought aboutat the sliding faces and local wear or seizure of the sliding faces maybe caused.

It is an object of this invention to resolve such a problem.

Specifically, it is an object of this invention to provide a pocket of ahydrostatic bearing with a sufficiently high pressure for preventingsolid contact of the sliding faces in a region where switching from thesuction stroke to the discharge stroke is performed.

DISCLOSURE OF THE INVENTION

A hydraulic pump according to this invention comprises a cylinder blockrotatably supported in a housing, a plurality of cylinders arranged on acircle a center of which coincides with a center axis of the cylinderblock, pistons respectively reciprocating in the cylinders, a suctionport and a discharge port provided to the housing selectivelycommunicating with the respective cylinders in accordance with arotation position of the cylinder block, a drive shaft inclinedrelatively to the center axis of the cylinder block, a rotationtransmitting mechanism for transmitting rotation of the drive shaft tothe cylinder block so as to rotate the cylinder block synchronously withthe drive shaft, a rotating disk rotating together with the drive shaftand cooperatively engaged with the pistons, a fixed sliding contact faceformed in the housing, the fixed sliding contact face being brought intosliding contact with a rear face of the rotating disk, pressure pocketsformed on the rear face of the rotating disk in correspondence withpositions of the respective pistons, and pressure paths for conductinghydraulic pressure in the respective cylinders to the respectivepressure pockets via the pistons.

The hydraulic pump further comprises a pressure introducing mechanismfor introducing a high pressure fluid to a pocket moving along the fixedsliding contact face along with the rotating disk at a position in whicha corresponding cylinder is about to switch from a suction stroke to adischarge stroke.

According to an aspect of this invention, the pressure introducingmechanism comprises a communication groove formed on a surface of thefixed sliding contact face for connecting the pocket at the position inwhich the corresponding cylinder is about to switch from the suctionstroke to the discharge stroke, and an adjacent pocket corresponding toa cylinder in the discharge stroke.

It is preferable that an orifice is provided midway along thecommunication groove.

According to another aspect of this invention, the pressure introducingmechanism comprises a fluid path communicating with the discharge portand having an opening in the fixed sliding contact face to communicatewith the pocket at the position in which the corresponding cylinder isabout to switch from the suction stroke to the discharge stroke.

It is also preferable that the fluid path is provided with an orifice.

According to yet another aspect of this invention, the pressureintroducing mechanism comprises a communication groove formed on thefixed sliding contact face and extending in a radial direction forcommunicating with the pocket at the position in which the correspondingcylinder is about to switch from the suction stroke to the dischargestroke, and a fluid path for introducing high pressure from thedischarge port to the communication groove.

It is also preferable that the fluid path is provided with an orifice.

According to this invention, when the drive shaft is rotated, thecylinder block is rotated, the pistons are reciprocated in therespective cylinders, a working fluid is sucked from the suction port toexpanding cylinders and the working fluid is discharged from contractingcylinders to the discharge port. Although a force corresponding to innerpressure of the contracting cylinders is exerted on the rotating disk aspiston reaction force, the force is supported by a hydrostatic bearingconstituted between the rotating disk and the fixed sliding contactface.

It is necessary that support force of respective pockets of thehydrostatic bearing is made to correspond to the force received from thecorresponding pistons. Therefore, when the inner pressure of a cylinderis switched from suction pressure to discharge pressure, pressure of thecorresponding pocket should be switched without delay.

The working fluid at high pressure is introduced to the pocketimmediately before the cylinder is switched from the suction stroke tothe discharge stroke by the pressure introducing mechanism. Therefore,the pressure of the pocket responses without being delayed whenswitching of the cylinder inner pressure takes place, and thehydrostatic bearing achieves always pertinent support force. As aresult, excessively large frictional force is not produced between therotating disk and the fixed sliding contact face, wear or seizure of thesliding faces does not occur, and the durability of the pump isenhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a hydraulic pump to which this inventionis applied;

FIG. 2 is an enlarged view of a portion of a piston;

FIG. 3 is a front view of a thrust plate according to a first embodimentof this invention;

FIG. 4 is a sectional view taken along a line A—A of FIG. 3;

FIG. 5 is a front view of a similar thrust plate according to a secondembodiment of this invention;

FIG. 6 is a sectional view taken along a line B—B of FIG. 5;

FIG. 7 is a front view of a thrust plate according to a third embodimentof this invention;

FIG. 8 is a sectional view taken along a line C—C of FIG. 7; and

FIG. 9 is a sectional view of a hydraulic pump featuring a thrust plateaccording to a fourth embodiment of this invention.

PREFERRED EMBODIMENTS

In this embodiment, the invention is applied to an axial piston pump,and as shown by FIG. 1, a pump housing 11 comprises a cylindrical case11C gripped by a side block 11A and a port block 11B.

A pump drive shaft 12 penetrating the side block 11A is rotatablysupported by a bearing 13. A cylinder block 14 is arranged at an innerspace of the pump housing 11. A rotation shaft 15 supported by the portblock 11B is inserted into the center of the cylinder block 14 via abearing 16 so as to allow the rotation the cylinder block 14 about therotating shaft 15 as center.

The cylinder block 14 is inclined to the pump drive shaft 12 by acertain angle such that axis centers of the pump drive shaft 12 and therotating shaft 15 intersect with each other. In order to transmitrotation of the drive shaft 12 to the cylinder block 14, the drive shaft12 and the cylinder block 14 are connected via a joint 17.

Spline heads 17C are formed at both ends of the joint 17, and insertedinto a spline hole 17A formed at an end face of the drive shaft 12 and aspline hole 17B similarly formed at a center of an end face of thecylinder block 14. In these holes, the joint 17 is spline jointed to thedrive shaft 12 and cylinder block 14. Outer peripheries of the splineheads 17C are formed in spherical faces so as to always maintainexcellent mesh and transmit rotation from the drive shaft 12 to thecylinder block 14 even when the center axes of the spline holes 17A and17B intersect with each other.

The cylinder block 14 is provided with a plurality of cylinder bores 18which are disposed at equal intervals on a circle about the rotatingshaft 15 as center. Center axes of the cylinder bores 18 are in parallelwith the rotating shaft 15. A piston 20 is slidably accommodated in eachof the cylinder bores 18.

The piston 20 is urged in an elongating direction by a coil spring 21arranged in the cylinder bore 18. In order to prevent the spring 21 frombeing folded to bend, a spring support 22 is arranged inside the coilspring 21. The spring support 22 is disposed in the hollow piston 20 andan end portion thereof is fixed to the piston 20, thereby preventingbuckling of the spring 21 and preventing the spring 21 from beingbrought into contact with an inner periphery of the piston 20. Thespring support 22 is formed by a material having small friction.

A piston cover 23 in a tube-like shape formed by synthetic resin(engineering plastic) is fitted on an outer periphery of the piston 20and is fixed thereto by adhesion. With this construction, friction of asliding face of the cover 23 with the cylinder bore 18 is maintainedsmall. The piston cover 23 has a length equal to or larger than theeffective stroke length of the piston 20, and a flange portion 23Aformed at a front end thereof is engaged with an end of the hollowpiston 20. The piston cover 23 may be constituted by a high polymermaterial having small frictional coefficient. A reinforcement materialsuch as carbon fiber may be added to the polymer material.

A valve plate 25 which is brought into contact with a bottom face of thecylinder block 14 is fixed to the port block 11B. The valve plate 25 isprovided with a pair of kidney ports (not illustrated), i.e., a suctionport and a discharge port to which ports 18A formed in the cylinderblock 14 and communicating with the respective cylinder bores 18successively connect according to rotation of the cylinder block 14. Asa result, a working fluid is discharged from the cylinder bores as thepistons 20 contract the cylinder bores, and the working fluid is suckedinto the cylinder bores when the pistons 20 expands the same.

A discharge path and a suction path, not illustrated, are formed in theport block 11B and connected to the kidney ports.

Also as shown by FIG. 2, a plane 20A perpendicular to the center axis ofthe piston 20 is formed at a front end of the piston 20. The plane 20Ais fitted with a pad 27 formed by synthetic resin having smallfrictional coefficient. The rear face of the pad 27 is provided with aprojecting portion 27A which is fitted in a hole of the piston 20. Thecenter of the projected portion 27A is provided with a through hole 27Bto thereby communicate with the interior of the piston 20. Further, aflat support face 27C of the pad 27 is formed with a pocket 27D to whichthe cylinder inner pressure is conducted via the interior of the piston20.

A shoe 29 in a semispherical shape is brought into contact with the pad27. The shoe 29 is supported by a socket 32 fitted to a torque plate 31.The torque plate 31 corresponds to a rotating disk arranged around thepump drive shaft 12 on a side of the side block 11A.

The socket 32 is formed by synthetic resin having small frictionalcoefficient and is fitted to a recess portion 31 A formed in the torqueplate 31. The socket 32 is provided with a recess portion 32A in asemispherical shape and a spherical face 29B of the shoe 29 is rotatablyaccommodated in the recess portion 32A.

A diameter of a flat smooth face 29A of the shoe 29 is formed to beslightly larger or substantially the same as a diameter of the supportface 27C of the pad 27 and the flat smooth face 29A and the support face27C are brought into face contact with each other. As described above,with regard to the contact face, hydraulic pressure in the piston isconducted to the pocket 27D so as to constitute a hydrostatic bearing byfluid between the shoe 29 and the pad 27, support load by the hydraulicpressure, and reduce wear therebetween.

Further, the shoe 29 is formed with a through hole 29C starting from theflat smooth face 29A and ending at the spherical face 29B, the fluid isconducted from the pocket 27D of the pad 27 to a pocket 29D formed at aportion of the spherical face 29B so as to constitute a hydrostaticbearing and reduce wear of the contact faces.

A spline portion 12A formed on the outer periphery of the pump driveshaft 12 is engaged with the torque plate 31 via a spline hole 31Bformed at the center of the torque plate 31 so as to rotate the torqueplate 31 integrally with the drive shaft 12. Accordingly, the torqueplate 31 is rotated in a same direction as that of the cylinder block14. Therefore, the shoes 29 supported by the sockets 32 of the torqueplate 31 and the pistons which are brought into contact with the shoes29 via the pads 27, are rotated along a circle around the drive shaft 12as center while always maintaining substantially the same positionalrelationship among them.

The torque plate 31 is accommodated in a recess portion 33 in a ringshape provided in the side block 11A around the drive shaft 12. A thrustplate 35 in a similar shape is arranged at a bottom face of the torqueplate 31 and the thrust plate 35 formed by synthetic resin having smallfrictional coefficient is fixed to the side block 11A. The torque plate31 is formed with pockets 31C at a face thereof sliding on the thrustplate 35 and the hydraulic pressure is conducted thereto. The hydraulicpressure is conducted from the hydrostatic bearing formed by the shoe 29to the pocket 31C via a through hole 32C provided to the socket 32 and athrough hole 31D provided to the torque plate 31. Thereby, contact facesof the torque plate 31 and the thrust plate 35 are supported by thehydrostatic bearing and sliding friction is reduced.

Further, a bush 36 made of synthetic resin having small frictionalcoefficient is arranged on the outer periphery of the torque plate 31and pressurized fluid is conducted to sliding faces of the outerperiphery of the torque plate 31 and the inner periphery of the bush 36to thereby constitute a hydrostatic bearing and reduce weartherebetween. For that purpose, there is formed a pressure introducingpath 37 communicating with a pump discharge path at inside of the sideblock 11A and the pressurized fluid is conducted to a pocket 36Aprovided at the sliding faces of the bush 36 and the torque plate 31.

When the pump drive shaft 12 is rotated by a prime mover, notillustrated, the torque plate 31 is rotated together and the cylinderblock 14 is also rotated via the joint 17.

Since the cylinder block 14 is inclined relative to the torque plate 31,the distance between the cylinder block 14 and the torque plate 31facing each other changes as they rotates.

In a rotation position range where the distance between the cylinderblock 14 and the torque plate 31 increases after the position at whichthe distance therebetween has taken the smallest value, the piston 20expands the cylinder bore while maintaining contact between the piston20 and the shoe 29 by being pushed by the spring 21, and the workingfluid is sucked to the cylinder bore 18 via the port 18A. On the otherhand, in another rotation position range where the distance between thecylinder block 14 and the torque plate 31 decreases after the positionat which the distance therebetween has taken the largest value, thepiston 20 is pushed by the shoe 29 and the fluid in the cylinder bore 18is discharged from the port 18A. The fluid is sucked from the suctionpath and discharged to the discharge path by operation of the valveplate 25.

In this way, by rotating the cylinder block 14, the pistons 20reciprocate while maintaining in contact with the shoes 29 held by thetorque plate 31, and suction and discharge of the working fluid from andto the cylinder bore 18 is repeated to thereby function as the axialpiston pump.

Meanwhile, a force in the axial direction is exerted on the piston 20 inaccordance with pressure of the fluid in the cylinder bore 18 and theforce is supported by the torque plate 31 via the shoe 29. In this case,the torque plate 31 is not perpendicular to the center axis of thepiston 20 and is inclined thereto by a certain angle. Accordingly,reaction force from the torque plate 31 via the shoe 29 involves acomponent force in a direction perpendicular to the center axis of thepiston 20.

However, the piston 20 and the shoe 29 are always brought into contactwith each other by planes perpendicular to the center axis, i.e., thesupport plate 27C of the pad 27 fitted to the piston 20 is in contactwith the flat smooth face 29A of the shoe 29. Accordingly, almost noforce in the direction perpendicular to the center axis of the piston 20which is parallel to these contact faces is transmitted to the piston20. Therefore, the piston 20 is free from a lateral force acting in thedirection perpendicular to the center axis, and a face pressure actingon the inner surface of the cylinder bore 18 is very small.

Rotational torque of the pump drive shaft 12 is transmitted to thecylinder block 14 via the joint 17, as well as to the torque plate 31via the spline portion 12B. The cylinder block 14 rotates together withthe torque plate 31 and accordingly, the pistons 20 and the shoes 29rotate around the pump drive shaft 12 while maintaining substantiallythe same positional relationship therebetween. Therefore, no relativetorque acts between the pistons 20 and the shoes 29 at any rotationangle and also thereby, large lateral force is not exerted on thepistons 20.

Friction by the sliding faces of the piston 20 and the cylinder bore 18is produced mainly in accordance with the lateral force exerted on thepiston 20, and when the lateral force is reduced in this way, thefrictional force can be reduced accordingly. Further, the cover 23 ofsynthetic resin is fitted to the outer periphery of the piston 20 tothereby reduce sliding resistance at a face thereof in contact with thecylinder bore 18.

As a result, the frictional force of the face of the piston 20 slidingwith the cylinder bore 18 is reduced. Therefore, even when water is usedas the working fluid, wear of the sliding face is reduced and highdurability is achieved.

Further, the pad 27 of synthetic resin having small friction isinterposed between the piston 20 and the shoe 29 to thereby avoid metalcontact between the piston 20 and the shoe 29. Further, the pocket 27Dis formed in the pad 27, inner pressure of the cylinder bore 18 isconducted to the pocket 27D via the interior of the piston 20, and thehydrostatic bearing is constituted between contact faces of the pad 27and the shoe 29. Therefore, contact pressure therebetween is reduced bythe hydraulic pressure, and the wear can be reduced.

The contact pressure between the pad 27 and the shoe 29 becomes highwhen the piston 20 performs the discharge stroke and conversely, and itbecomes low when the piston performs the suction stroke. Therefore, thepressure required for the hydrostatic bearing becomes high in thedischarge stroke and low in the suction stroke. Since the inner pressureof the cylinder bore 18 is conducted to the pocket 27D via the piston20, the characteristics of the pressure provided to the pocket 27Dcoincides with those required for the hydrostatic bearing. The pocket27D, therefore, functions as an excellent hydraulic bearing.

Further, the socket 32 of synthetic resin is provided between the shoe29 and the torque plate 31 to thereby avoid metal contact therebetweenby preventing the shoe 29 and the torque plate 31 from being broughtinto direct contact with each other. Further, the hydraulic pressure isconducted to the spherical contact faces of the socket 32 and the shoe29 via the pocket 29D to thereby constitute the hydrostatic bearingbetween the respective contact faces. Therefore, also with regard tothese sliding faces, mechanical contact force is reduced and wear isreduced.

The torque plate 31 rotating together with the pump drive shaft 12suffers reaction forces of the pistons 20 in the discharge stroke, andis pushed towards the recess portion of the side block 11A in the thrustdirection and the radial direction in accordance with the inclination ofthe pistons 20. The torque plate 31 is supported by the thrust plate 35in the direction of the rotational axis thereof against the thrust forceand is supported by the bush 36 in the lateral direction against theradial force. Accordingly, under either of these forces, metal contactof the sliding faces is avoided. Further, between the contact faces ofthe torque plate 31 and the thrust plate 35 and between the contactfaces of the torque plate 31 and the bush 36, hydraulic pressure isconducted and the hydrostatic bearings are constituted respectively.Accordingly, mechanical contact between these members is reduced, wearof the torque plate 31 is reduced, and the durability is enhanced.

In this way, the frictional forces are reduced and wear is reduced withregard to the sliding faces of the piston 20 and the shoe 29, thespherical sliding faces of the shoe 29 and the torque plate 31, andthrust and radial sliding faces of the torque plate 31 and the sideblock 11A. In this axial piston pump, therefore, high durability can beensured even when water, which is poor in lubrication performance, isused as the working fluid.

As shown in FIG. 3 and FIG. 4, the thrust plate 35 is provided with ahole 35A for inserting the drive shaft 12 at its center and providedwith a flat sliding contact face 35B around the hole 35A. The slidingface 35B is brought into sliding contact with the rear face of thetorque plate 31. A plurality of the pockets 31C of the torque plate 31shown by imaginary lines move in a direction indicated by an arrow markin accordance with rotation of the drive shaft 12, and positions thereofrelative to the sliding contact face 35B of the thrust plate 35 arechanged.

The cylinders in correspondence with the pockets 31C communicate withthe suction port or the discharge port depending on the rotationalpositions. In FIG. 3, the respective pockets 31C (in) arranged on theleft half side of the thrust plate 35 in the figure corresponds to asuction region wherein the corresponding cylinders communicate with thesuction port. On the other hand, the respective pockets 31C (out)arranged on the right half side of the thrust plate 35 in the figurecorresponds to a discharge region wherein the corresponding cylinderscommunicate with the discharge port.

A communication groove 40 extending in the circumferential direction forconnecting the two pockets 31C (in) and 31C (out) is formed at aposition where the pocket 31C is switched from the suction region to thedischarge region on the surface of the sliding face 35B of the thrustplate 35.

With this construction, the pocket 31C (in) is operated with highpressure from the pocket 31C (out) disposed already in the dischargeregion immediately before the connection of the corresponding cylinderis switched from the suction port to the discharge pressure. Therefore,support force of the thrust hydrostatic bearing constituted by thepocket 31C (in) is increased without delay to the increase in the innerpressure of the corresponding cylinder.

In this way, pressure in the pocket 31C is increased without delay tothe change of the inner pressure of the cylinder, and the hydrostaticbearing always achieves pertinent support force. Accordingly,excessively large friction is not produced between the sliding faces ofthe torque plate 31 and the thrust plate 35, relative rotationtherebetween is smooth, so wear or seizure is prevented from causing andthe durability of the pump is enhanced.

The shape of the communication groove 40 is not particularly limited.The number of the communication grooves 40 is also not limited to singlebut can be plural. Although the hydrostatic bearing is formed betweenthe torque plate 31 and the thrust plate 35 in this embodiment, it isalso possible to eliminate the thrust plate 35, the torque plate 31 isbrought into direct contact with the side block 11A, and the hydrostaticbearing is formed therebetween. In this case, the communication groove40 is formed directly on a sliding face of the side block 11A.

FIG. 5 and FIG. 6 show another embodiment of this invention. Accordingto this embodiment, the thrust plate 35 has a communication groove 41having a small sectional area functioning as an orifice. Thecommunication groove 41 has a V-shape cross section, and the sectionalarea thereof gradually changes according to a distance from the pockets.The maximum sectional area is obtained at the middle portion. With thisconstruction, flow of high pressure fluid conducted from the pocket 31C(out) to the pocket 31C (in) is throttled and the flow rate ispertinently restricted. Thereby, leakage of the high pressure fluid isprevented from becoming excessively large, so the pump efficiency isprevented from becoming low.

FIG. 7 and FIG. 8 show still another embodiment of this invention.

According to this embodiment, a path 44 penetrating the thrust plate 35is provided and connected to the discharge port via the pressureintroducing path 37 passing through the wall of the pump housing 11.

An opening 44A at the thrust plate sliding contact face 35B of the path44 is formed at a position slightly deviated from a neutral positiontowards the discharge region (right half side of FIG. 7) such that thedischarge pressure is conducted to the pocket 31C (in) immediatelybefore the pocket 31C (in) is moving from the suction region to thedischarge region.

In this case, high pressure can always be conducted to the pocket 31C(in) which is about to move from the suction region to the dischargeregion via the path 44 and the function of the proper hydrostaticbearing is achieved similar to the aforesaid first and secondembodiments. Further, when an orifice or chalk is provided midway alongthe path 44, the flow rate introduced to the pocket 31C can pertinentlybe controlled.

Still another embodiment of this invention will be described byreferring to FIG. 9.

This embodiment relates to the hydrostatic bearing for conducting pumpdischarge pressure via the pressure introducing path 37 between theouter peripheral face of the torque plate 31 and the recess portion 33of the side block 11A. Higher pressure is conducted from the pressureintroducing path 37 to a vertical pocket 45 on the inner surface of therecess portion 33. The high pressure is then conducted to communicationgroove 46 provided on the sliding contact face 35B of the thrust plate35.

In this case, a communication groove 46 is formed in the radialdirection while being inclined slightly from the neutral position of thesliding contact face 35B to the discharge region. Thereby, thecommunication groove 46 communicates with the pocket 31C (in) when thepocket moves from the suction region to the discharge region so as tointroduce the high pressure fluid into the pocket 31 c (in). In thisway, high pressure is introduced with no delay of response when a pocket31 moves from the suction region to the discharge region and pertinentfunction of the hydrostatic bearing is maintained.

What is claimed is:
 1. A hydraulic pump comprising: a cylinder blockrotatably supported in a housing; a plurality of cylinders arranged on acircle a center of which coincides with a center axis of the cylinderblock; pistons respectively reciprocating in the cylinders; a suctionport and a discharge port provided to the housing selectivelycommunicating with one end of the respective cylinders in accordancewith a rotational position of the cylinder block; a drive shaft inclinedrelative to the center axis of the cylinder block; rotation transmittingmeans for transmitting rotation of the drive shaft to the cylinder blockto thereby rotate the cylinder block synchronously with the drive shaft;a rotating disk rotating together with the drive shaft and cooperativelyengaged with the pistons at another end of the respective cylinders; afixed sliding contact face formed in the housing, the fixed slidingcontact face being brought into sliding contact with a rear face of therotating disk; pressure pockets formed on the rear face of the rotatingdisk in correspondence with positions of the respective pistons;pressure paths for conducting hydraulic pressure in the respectivecylinders to the respective pressure pockets via the pistons; andpressure introducing means for introducing a high pressure fluid to apocket moving along the fixed sliding contact face along with therotating disk at a position in which a corresponding cylinder is aboutto switch from a suction stroke to a discharge stroke.
 2. The hydraulicpump as defined in claim 1, wherein the pressure introducing meanscomprises a communication groove formed on a surface of the fixedsliding contact face for connecting a pocket at the position in whichthe corresponding cylinder is about to switch from the suction stroke tothe discharge stroke, and an adjacent pocket corresponding to a cylinderin the discharge stroke.
 3. The hydraulic pump as defined in claim 2,wherein an orifice is provided midway along the communication groove. 4.The hydraulic pump as defined in claim 1, wherein the pressureintroducing means comprises a fluid path communicating with thedischarge port and having an opening in the fixed sliding contact faceto communicate with a pocket at the position in which the correspondingcylinder is about to switch from the suction stroke to the dischargestroke.
 5. The hydraulic pump as defined in claim 4, wherein the fluidpath is provided with an orifice.
 6. The hydraulic pump as defined inclaim 1, wherein the pressure introducing means comprises acommunication groove formed on the fixed sliding contact face forcommunicating with a pocket at the position in which the correspondingcylinder is about to switch from the suction stroke to the dischargestroke and extending in a radial direction, and a fluid path forintroducing high pressure from the discharge port to the communicationgroove.
 7. The hydraulic pump as defined in claim 6, wherein the fluidpath is provided with an orifice.